The present invention is directed to an apparatus and method for controlling an automatic transmisson for an automobile. More specifically the present invention relates to an apparatus and method for controlling an automatic transmission in the event of an interruption or loss of the vehicle velocity signal.
Conventional automatic transmission control apparatus have a control circuit which gives a shift signal to shift the transmission in response to the vehicle velocity and the output torque of an engine or throttle opening signal. The vehicle velocity signal is supplied from a speed sensor with a reed switch which detects the revolution speed of an output shaft. The speed sensor is comprised of a magnet rotated by the output shaft for a speedometer cable which transmits rotary motion from the output shaft to a speedometer. The reed switch opens and closes under the influence of the rotating magnet to output pulses at a frequency related to the velocity of the vehicle. The speed sensor is positioned adjacent to the output shaft or in the speedometer and has its wires connected to the control circuit. With such a speed sensor, the reed switch or the wires may be broken to cause an interruption or loss of the vehicle speed signal. Ordinarily this produces the same effect as a vehicle at rest and the control circuit gives a signal to shift the transmission downwardly to a low gear. This may not be a problem at low vehicle speed but it causes violent braking which jolts the driver and passengers and may lead to engine or transmission failure at higher vehicle speeds.
U.S. Pat. No. 4,499,542 discloses a safety circuit which responds to the interruption or loss of the vehicle velocity signal to control the transmission to maintain a high speed gear ratio. After the vehicle velocity signal is restored, the safety control circuit allows the transmission to change gears. However this safety control circuit maintains the same high speed gear ratio unless the vehicle velocity signal is restored. Therefore usual driving of the vehicle will be impossible once the speed sensor breaks down.
Japanese Patent Application No. 59-252,500, filed Nov. 29, 1984, which corresponds to pending U.S. patent application Ser. No. 802,399, and is assigned to the same assignee as the present application, discloses a control apparatus which detects an apparent interruption or an abrupt loss of vehicle speed due to failure of the speed sensor and shifts the automatic transmission to a higher rear ratio. The control apparatus detects the engine rotation and the position of the shift lever so that when the engine rotation is under a predetermined level the control apparatus allows the automatic transmission to be shifted by the position of the shift lever. Thus the pending application allows the shifting of the automatic transmission even in the absence of a vehicle speed signal to provide responsible driving conditions at all times. According to the arrangement of this pending application, an apparent abrupt loss of the vehicle speed as indicated by the speed sensor is detected by the steps as follows: a counter circuit of the micro-computer counts pulses from the speed sensor within a time interval; the micro-computer calculates the vehicle speed; if the current vehicle speed is "0" the computer refers the previous vehicle speed; and if the previous vehicle speed is higher than a predetermined value the computer decides the speed change is an abrupt one. These steps also detect an apparent abrupt loss of the vehicle speed as indicated by the speed sensor in the event of wheel locking which may be caused by a brake operation. However, under the wheel locking condition the speed sensor may not have failed.